August 2001, VOLUME 177
NUMBER 2

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August 2001, Volume 177, Number 2

Gastrointestinal Imaging

Prevalence of Incidental Bochdalek's Hernia in a Large Adult Population

+ Affiliation:
1All authors: Department of Radiology, Harvard Medical School, Boston, MA 02114, and Department of Radiology, Founders House, Massachusetts General Hospital, Boston, MA 02114.

Citation: American Journal of Roentgenology. 2001;177: 363-366. 10.2214/ajr.177.2.1770363

ABSTRACT
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OBJECTIVE. The purpose of this study was to determine the prevalence and characteristics of adult Bochdalek's hernia in a large patient population.

MATERIALS AND METHODS. We retrospectively reviewed all abdominal CT scans obtained at our hospital in 1998. Patients in our study were identified through a keyword search of our database for “Bochdalek,” “hernia,” and “diaphragm.” The individual patient studies identified were reviewed in a soft-copy format. We noted the location and side of the body on which the diaphragmatic hernia arose and the contents of the sac. We also performed a chart review for each patient included in the study, noting the patient's sex, age, and symptoms.

RESULTS. Incidental Bochdalek's hernia was diagnosed in 22 patients (17 women, five men), which represents an incidence of 0.17% based on 13,138 abdominal CT reports we reviewed. The mean age of the patients was 66.6 years. None of the patients were symptomatic. Sixty-eight percent of the hernias were on the right side of the body, 18% were on the left side, and 14% were bilateral. Seventy-three percent contained only fat or omentum, whereas 27% had solid or enteric organ involvement including the spleen, small intestine, or large intestine.

CONCLUSION. Bochdalek's hernia is not rare, and the incidence of Bochdalek's hernias that contain enteric tract is higher than previously reported. This incidence likely represents a conservative estimate because some Bochdalek's hernias may have been overlooked or unreported.

Introduction
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Congenital hernias resulting from a developmental failure of posterolateral diaphragmatic foramina to fuse properly were first described by Bochdalek in 1848 [1], although the origins of descriptions of diaphragmatic hernia can be dated to writings of 1690 [2]. Most Bochdalek's hernias are diagnosed in children, ranging from neonates through preschoolers, who present with clinical symptoms caused by associated pulmonary insufficiency [1,2,3,4,5,6,7,8].

In classic cases, Bochdalek's hernias typically arise on the left side, contain fat or omentum predominantly and the kidney rarely, and do not necessarily lead to symptoms. Varying in size and contents, they may be found in patients of any age. Identification of previously undiagnosed Bochdalek's hernia in adults occurs most frequently when the patients are undergoing CT for reasons that appear to be unrelated to the (frequently asymptomatic) hernia—typically for surveillance of cancer or assessment of vague symptoms.

Approximately 100 cases of occult Bochdalek's hernias in asymptomatic adults have been reported in the literature. However, the true prevalence of Bochdalek's hernia remains unknown, with estimates ranging from a low of 1 in 2000-7000 based on autopsy studies [1, 4] to a high of 6% based on the findings of early-era CT examinations [5]. Moreover, the reports describing the characteristics of incidental Bochdalek's hernias in adults have been based on the findings in small groups of patients and on anecdotal information [3, 6, 8,9,10]. It may be suspected that at least some patients presenting late with incidental Bochdalek's hernia may have acquired herniation during adulthood. The question of treatment of Bochdalek's hernia remains controversial but is important because hernia contents may be large and contain parenchymal or tubular organs that may become entrapped and compromised [11].

We hypothesized that with routine use of thin-section CT scanning on modern imaging equipment, the prevalence and characteristics of late-presenting Bochdalek's hernia could be more accurately estimated [12,13,14]. (Specifically, a study by Killeen et al. [14] published in 1999 found CT sensitivities of 78% for left-sided hernia and 50% for right-sided hernia.) In addition, we sought to better define the entity and incidence of incidental Bochdalek's hernia using current technological tools because hernia contents of abdominal organs can threaten the patient's well-being, because published values for the condition vary, and because there is a relative dearth of recent literature on the topic.

Materials and Methods
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We retrospectively queried a database containing the radiology reports of abdominal CT examinations performed during the calendar year 1998 (n = 13, 138) using any or all of the following keywords: “Bochdalek,” “hernia,” and “diaphragm.” We used a commercially available radiology information system software (Folio; Camberly Systems, Boston, MA). The investigation was performed at a single institution, a large urban tertiary care hospital.

We defined a Bochdalek's hernia as a posterolateral diaphragmatic interruption (Fig. 1). It was differentiated from eventration by direct visualization of a diaphragmatic interruption or foramen located in the posterolateral position. The finding of fat above the diaphragm was not the only diagnostic criterion because fat may exist cephalad to an intact diaphragm [5]. In selected cases, 5-mm axial CT scans were reformatted into 1-mm axial, sagittal, and coronal images for calrification. More than 50 candidates were identified solely by the keyword search. Patients who met the criteria were included in the cohort, and all others were excluded. Hernia dimensions were not routinely obtained because our requesting physicians value estimations of hernia size (small, moderate, or large) more than volume of the hernia sac.

figure
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Fig. 1. Unilateral Bochdalek's hernia in 63-year-old woman. Axial CT scan with lung windows shows small fatcontaining left-sided Bochdalek's hernia (straight arrow). No solid organs are contained within hernia. Posterolateral location of diaphragmatic interruption (curved arrow) is easily seen.

To obtain a clinical history for the patients in whom Bochdalek's hernia was identified, we performed a chart review using their hospital records, including discharge summaries, operative notes, and radiology reports. Only patients recently diagnosed with cases of the disease were included in the study. None of the patients had experienced a recent trauma (excluding childbirth). The predominant clinical indication for obtaining the abdominal CT scans was to exclude the possibility of metastatic disease in patients with a known malignancy. Bochdalek's hernias were identified in both inpatients and outpatients.

For this analysis, the patients' thoracic CT scans were not included because the routine protocol for chest CT scans consisted of 10-mm contiguous slices, whereas the protocol for abdominal CT scans consisted of 5-mm contiguous slices. We reviewed the CT scans of patients with reports indicating a Bochdalek's hernia individually. Twenty-two patients were identified, 17 women and five men. Correlation with abdominal or chest radiographs was not performed routinely.

CT scans were obtained on a helical scanner (HiSpeed CTI; General Electric Medical Systems, Milwaukee, WI). A routine abdominal CT scan protocol consisted of 5-mm collimation with a pitch of 1.5-1.8 from just above the diaphragm to the pelvic inlet. Either IV or oral contrast agents were administered, depending on the clinical indication. Images were reviewed in a soft-copy format with lung and soft-tissue settings on a picture archiving and communications system workstation (IMPAX; Agfa Division, Bayer, Ridgefield Park, NJ).

Among the data elements evaluated were the patient's sex, age, and presenting symptoms; the location and contents (fat only or intraabdominal organs) of the Bochdalek's hernia; and the side of involvement (right, left, or bilateral). None of the patients underwent surgery during the acute interval (during the same admission) after the study had been performed.

Results
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We diagnosed incidental Bochdalek's hernias in 22 patients on the basis of radiology case reviews (Figs. 1,2,3). In each instance, the patient's symptoms were not directly referable to the site of, or contents within, the hernia, and so we deemed the finding of the hernia to be incidental. More than half of the patients were being evaluated for cancer staging. In each case, there was no evidence in the patient's medical record that the hernia had been previously identified. These 22 patients represented an incidence of 0.17%, based on the 13,138 abdominal CT scans performed during 1998. We believed the hernias were of small or moderate size in most patients.

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Fig. 2. Bilateral Bochdalek's hernia in 74-year-old woman. Axial CT scan with soft-tissue windows shows bilateral Bochdalek's hernias (arrows). Left hernia contains fat and portions of spleen; right hernia contains fat only.

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Fig. 3. Bochdalek's hernia containing stomach and pancreas in 29-year-old woman. Axial CT scan with soft-tissue windows shows fat, pancreas, and enteric tract (stomach) containing left-sided Bochdalek's hernia (arrow). There was no intestinal obstruction. Posterolateral location of diaphragmatic interruption is easily seen.

We found Bochdalek's hernia more frequently in women patients than in men patients by a ratio of 17:5 (77% women). The mean patient age was 66.6 years, and the age range was between 29 and 91 years old. Sixty-eight percent (15/22) of the hernias were on the right side, 18% (4/22) were on the left (Figs. 1 and 3), and 14% (3/22) were bilateral (Fig. 2). Seventy-three percent (16/22) of the Bochdalek's hernias contained intraabdominal fat or omentum only, whereas 27% (6/22) had solid or enteric organ involvement including the stomach, liver, spleen, pancreas, or kidney. The kidney (50%, 3/6 patients with organ involvement) was the most common parenchymal organ to be involved in a Bochdalek's hernia, and the spleen (33%, 2/6 patients) was the second most common. Involvement of the stomach, liver, or pancreas was visualized on the scans of one patient each (Fig. 3). Some patients had more than one abdominal organ contained at least partially within the hernia (Fig. 3). All of the Bochdalek's hernias contained at least some fat or omentum.

Discussion
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The location of the foramina of Bochdalek is defined by the location of the diaphragmatic coronary ligaments bilaterally. Bochdalek's hernia occurs when these soft-tissue anastomoses fail to close or when they reopen. If the herniation is present from the time of birth, it is termed “congenital.” If the herniation forms later, perhaps because of extension of intraabdominal or perirenal fat into the thorax, it is termed “acquired.” Acquired hernias are also called “incidental” or “subacute” hernias.

Bochdalek's hernia most commonly manifests during the patient's first few weeks of life. Diagnosis beyond the first 8 weeks of life is estimated to represent 5-25% of all Bochdalek's hernias [4]. In the neonate, Bochdalek's hernia is one of the leading causes of respiratory distress and remains one of the most common congenital anomalies of the thorax [15]. Most neonatal Bochdalek's hernias are left-sided [16].

In adults, most Bochdalek's hernias are likely to be asymptomatic, and thus the finding of the condition is incidental [2, 7]. The symptoms are typically vague in those patients who do experience them at the time of diagnosis. The patients may present with chest pain or describe symptoms that are generally referable to the gastrointestinal tract [2, 7].

Putative causes for late-presenting hernias include congential herniation, blunt or penetrating trauma, physical exertion (including sexual intercourse), pregnancy, labor and delivery, sneezing or coughing, and even ingestion of a large meal [1]. If patients with latepresenting hernias typically had previous CT or MR imaging studies that had been obtained over time, the causes of individual cases of hernia and the evolution of this entity could be discerned. Unfortunately, in most cases—as with all the patients in this investigation—no CT or MR images obtained before presentation and no follow-up images exist for comparison. Sener et al. [8] made a distinction between large and small hernias and noted that in most patients Bochdalek's hernias are small and contain only a modest amount of fat.

Diaphragmatic rupture occurs in as many as 25% of blunt thoracic and abdominal traumas [17], and traumatic diaphragmatic rupture is more common on the left side of the body [18]. Such injury may be diagnosed initially, but it may also go undiagnosed [19]. Some of the undiagnosed patients may never be identified as having Bochdalek's hernia, whereas others will present with symptoms related to the herniation (possibly with strangulation of abdominal contents) [20, 21]. Other patients will go on to have the hernia identified on images performed for other, unrelated indications. None of the patients in this cohort had a history of trauma (other than childbirth) documented in the medical records.

The literature reports a left-sided predominance for Bochdalek's hernia, with left-sided occurrence of the hernia accounting for 70-90% of cases [1, 4, 5]. These hernias may contain fat, retroperitoneal structures, or intraperitoneal contents, although the latter two conditions are exceedingly rare [6]. In the instance of intrathoracic abdominal organs, a peritoneal sac is noted. The size of the hernia seen on cross-sectional imaging does not necessarily correspond to the size of the diaphragmatic defect, which may be substantially larger [6]. The incidence of hernia with peritoneal sac varies from 10-38% [1]. In right-sided Bochdalek's hernias, the contents are predominantly the liver, the kidney, and fat. A left-sided hernia may contain enteric tract, the spleen, the liver, the pancreas, the kidney, or fat. Colon-containing hernias are rare and usually occur through left-sided defects [3]. Bilateral Bochdalek's hernias were found by Gale [5] to represent a prevalence of 3-6%. Of note, congenital Bochdalek's hernia is usually an isolated anomaly [4], a contradistinction to many other congenital defects in which multiple anomalies are found together [4].

In this study, we identified 22 patients with incidental Bochdalek's hernias. A study by Gale [5] published in 1985 described 52 cases of Bochdalek's hernia in 940 adult patients, a prevalence of 6%. The patients described by Gale represent a prevalence, and our study represents an incidence. It is unclear why the values of our large study and other smaller studies differ so significantly from those of Gale. Possibilities include selection bias and differences in inclusion criteria, such as the relatively poorer resolution of CT in 1985 and possible inclusion of patients with diaphragmatic bosselation. Two studies appearing after that of Gale describe incidences of less than 1%, which more closely match our results [1], 3]. Our findings suggest that Bochdalek's hernias are more common than reported in the literature—with the singular exception of the study by Gale. Other possible explanations for this observation would include an increase in the number of abdominal CT scans ordered by today's primary care providers, increased diagnostic sensitivity attributable to technical improvements, and, possibly but less likely, an increasing incidence of posterolateral diaphragmatic hernias.

Our results also indicate that right-sided Bochdalek's hernias are more common than has been reported in the literature. The reason for this finding is unclear, and the “protection” previously believed to be offered by the space-occupying liver offered was apparently overestimated. A relatively large number of the patients (27%) had Bochdalek's hernias that contained enteric or solid organs, a contradistinction to the numbers reported in the literature.

Our study results show a predominance of women with Bochdalek's hernias and a much higher prevalence of bilateral hernias than has been described in the literature. We diagnosed bilateral Bochdalek's hernias, an entity thought to be uncommon, in three patients. It is likely that large bilateral defects are incompatible with life. Thus, only small late-presenting defects would be asymptomatic (consistent with the findings in these patients).

Our investigation has limitations. Our results represent a conservative estimate of the actual incidence of Bochdalek's hernia. We relied on a retrospective review of case records using keyword searches to identify candidates. Certainly, some cases of Bochdalek's hernia were missed, others were noted and not mentioned, and still others may have been noted and misdiagnosed as another condition. In addition, we were limited by the lack of operative or pathologic data for our standards. Thus, despite our best efforts, some of these patients may indeed not have had Bochdalek's hernia. We are contemplating a prospective consecutive series of CT scans for the identification of Bochdalek's hernia, which would likely reduce error in identification of small potential herniations that would otherwise not be reported.

The clinical importance of these findings is threefold. The physical characteristics of incidental or late-presenting Bochdalek's hernia may confound diagnosis of findings at physical examination or chest radiology, specifically a higher-than-previously-thought incidence of right-sided Bochdalek's hernia. Secondly, women are more at risk for this entity than previously described. Thirdly, involvement of the intraabdominal organs in Bochdalek's hernias is more common than previously described, and this involvement may lead to symptomatology and possibly necessitate further radiologic examination or surgical intervention.

In summary, these findings indicate that Bochdalek's hernias are likely more common in the population of the United States than has been reported. The incidences of both Bochdalek's hernias containing enteric tract organs and bilateral Bochdalek's hernias are much higher than previously reported. Women with Bochdalek's hernias predominated in our study, a contradistinction to reports in the literature.

Presented at the annual meeting of the Radiological Society of North America, Chicago, IL, November 1999.

Address correspondence to M. E. Mullins at Massachusetts General Hospital.

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